Concepts of Human Anatomy & Physiology   5/e   Van De Graaff/Fox
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Disorders of Muscle Tissue

Muscular

Muscle disorders are caused by disruption of normal innervation, degeneration and replacement of muscle cells, injury, lack of use or disease.

Atrophy

Muscular atrophy refers to a decrease in size of muscles. Individual muscle fibers decrease in size, and there is a progressive loss of myofibrils.

Disuse atrophy is a muscular atrophy that results from lack of muscle use. Bedridden people, people with limbs in casts, or those who are inactive for other reasons experience disuse atrophy in the muscles that are not used. Disuse atrophy is temporary if a muscle is exercised after it is taken out of a cast. However, extreme disuse of a muscle results in muscular atrophy in which there is a permanent loss of skeletal muscle fibers and the replacement of those fibers by connective tissue. Immobility that occurs in bedridden elderly people can lead to permanent and severe muscular atrophy.

Denervation atrophy results when nerves that supply skeletal muscles are severed. When motor neurons innervating skeletal muscle fibers are severed, the result is flaccid paralysis. If the muscle is reinnervated, muscle function is restored, and atrophy is stopped. However, if skeletal muscle is permanently denervated, it atrophies and exhibits permanent flaccid paralysis. Eventually muscle fibers are replaced by connective tissue, and the condition cannot be reversed.

Transcutaneous stimulators are used to supply electrical stimuli to muscles that have had their nerves temporarily damaged or to muscles that are put in casts for a prolonged period of time. The electrical stimuli keep the muscles functioning and prevent permanent atrophy of the muscles while the nerves resupply the muscles or until the cast is removed.

Muscular Dystrophy

Muscular dystrophy is one of a group of diseases called myopathies that destroy skeletal muscle tissue. Usually the diseases are inherited and are characterized by degeneration of muscle cells, leading to atrophy and eventual replacement by connective tissue.

Duchenne muscular dystrophy almost exclusively affects males, and by early adolescence the individual is confined to a wheelchair. As the muscles atrophy, they shorten, causing conditions such as immobility of the joints and postural abnormalities such as scoliosis. Duchenne muscular dystrophy is inherited as a sex-linked recessive gene. The structure of the abnormal gene has been determined, and the gene has been found to cause the production of an abnormal protein. The abnormal protein results in abnormal contractions and progressive muscular weakness. The normal gene causes the production of protein which regulates the activity of other proteins in the plasma membrane.

Facioscapulohumoral (fa'si-o-skap-u-lo-hu'mor-al) muscular dystrophy is generally less severe, and it affects both sexes later in life. The muscles of the face and shoulder girdle are primarily involved. Facioscapulohumoral muscular dystrophy appears to be inherited as an autosomal dominant condition. Both types of muscular dystrophy are inherited and progressive, and no drugs prevent the progression of the disease. Therapy primarily involves exercises. Braces and corrective surgery sometimes help correct abnormal posture caused by the advanced disease.

Research is directed at identifying the genes responsible for all types of muscular dystrophy, exploring the mechanism that leads to the disease condition, and finding an effective treatment once the mechanism for the disease is known.

Fibrosis

Fibrosis is the replacement of damaged cardiac muscle or skeletal muscle by connective tissue. Fibrosis, or scarring, is associated with severe trauma to skeletal muscle and with heart attack (myocardial infarction) in cardiac muscle.

Fibrositis

Fibrositis is an inflammation of fibrous connective tissue, resulting in stiffness, pain, or soreness. It is not progressive, nor does it lead to tissue destruction. Fibrositis can be caused by repeated muscular strain or prolonged muscular tension.

Cramps

Cramps are painful, spastic contractions of muscles that usually result from an irritation within a muscle that causes a reflex contraction. Local inflammation resulting from a buildup of lactic acid and fibrositis causes reflex contraction of muscle fibers surrounding the irritated region.

Fibromyalgia, or chronic muscle pain syndrome, has muscle pain as its main symptom. Fibromyalgia has no cure, but it is not progressive, crippling, or life-threatening. The pain occurs in muscles or where muscles join their tendons, but not in joints. The pain is chronic and widespread, and it is distinguished from other causes of chronic pain by the identification of tender points in muscles, by the length of time the pain persists, and by failure to identify any other cause of the condition.

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